1. Field of the Invention
The present invention relates to an organic electroluminescence element (may also referred to as an organic EL element, hereinafter).
2. Description of the Related Art
Organic electroluminescence elements using organic compounds have drawn the attentions of people in the art due to their great potential to be used as inexpensive, large, full-color solid luminescence display elements, or writing light arrays, and hence various studies and researches have been conducted on them. Generally, an organic electroluminescence element is composed of a couple of counter electrodes and a light-emitting layer disposed between the electrodes. Once an electric field is applied to both electrodes, electrons are injected from a cathode and holes are injected from an anode. These electrons and holes are recombined in a light-emitting layer, and when the electrons return from a conduction band to a valence band, energy is released from the electrons as light. In this manner, light is emitted from an organic electroluminescence element.
Conventional organic electroluminescence elements require high driving voltages, and have low luminance of the emitted light and emission efficiency. However, various technologies have recently been reported to solve these problems.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 2006-121032, there is disclosed an organic electroluminescence element that includes a light-emitting layer containing 4,4′-N,N′-dicarbazole-biphenyl (CBP) as a host material, a platinum complex as a blue phosphorescent light-emitting material, and a platinum complex as a red phosphorescent light-emitting material.
It is generally known that the platinum complex (the platinum complex with 4 ligands) has a two-dimensional structure. When the platinum complex is used, molecules of the platinum complex were stacked to each other. Therefore, the distance between the molecules is short compared to cases when an iridium complex is used.
Moreover, it is considered that the shorter the distance between the molecules is, the more easily interaction between the molecules occurs. Therefore, if the blue platinum complex is used in combination with the red platinum complex, the distance between molecules thereof is shorter, resulting in a light-emitting layer in which energy is easily transferred.
For this reason, when two platinum complexes are used as a blue phosphorescent light-emitting material and red phosphorescent light-emitting material, energy is easily transferred to the red phosphorescent light-emitting material compared to the case where a combination of a platinum complex and iridium complex is used. In this case, the resulted organic electroluminescence element does not emit white light unless an amount of the red phosphorescent light-emitting material contained therein is significantly reduced.
Moreover, in Proceeding of ASID '06 pp. 50-52, there is disclosed a white organic electroluminescence element that includes a light-emitting layer doped with phosphorescent light-emitting compounds such as iridium (III) bis(4,6-difluorophenyl)-pyridinate-N, C2)picolinate(Firpic), and bis(2-(2′-benzo[4,5a]-thienyl)pyridinate-N,C3′)iridinium(acetylacetonate)(Ir(btp)2(acac)).
However, the organic electroluminescence element disclosed in Proceeding of ASID '06 pp. 50-52 has a problem in endurance as it uses two Ir complexes.